Fluorescent markers have been used to differentiate between diseased and healthy tissues in connection with applying therapies such as surgical excision. Photodynamic substances have been used with properties that cause them to accumulate in tumor cells so that the fluorescence can be used to detect the cancer cells. One such substance is 5-aminolevulinic acid (5-ala) which is taken up by all cells but which is quickly eliminated by healthy cells but converts to a fluorescent substance protoporphyrin IX in cancer cells. During surgery to remove a tumor, excitation light is provided to the tissues, and the resulting fluorescent areas mark the cancer for removal. Another known photo-sensitizer substance is porfimer sodium, sold as Photofrin® by Axcan Pharma of Birmingham, Ala.
Due to the low concentrations of the photo-sensitive substance in the tissues, the amount of fluorescent light produced in the diseased tissues may be low. Thus, it may be difficult for a surgeon to see all the areas containing the marker substance. In surgeries of certain organs, such as the brain, it is desirable to ascertain precise boundaries between cancerous and healthy tissues so that all cancerous tissue can be removed without affecting any healthy tissue. To improve recognition, electronic systems with higher sensitivity than the human eye have been developed for sensing the areas that fluoresce. A sensed image of the fluorescing areas has been presented on a display screen or monitor. However, such systems still have a drawback in that the surgeon must estimate where the areas depicted on the monitor are actually located in the patient when performing the tissue removal.